Jason Giambi hit Al Leiter’s 3-and-2 third-inning pitch so hard, the Met starter nearly suffered whiplash. And after allowing two more homers in the third inning last night, Leiter’s head must’ve surely been spinning.

A nanosecond after Giambi sent Leiter’s full-count offering towards Monument Park, Leiter jerked his head violently up and around to left-center.

The veteran lefty took a few sidesteps toward the third-base line to back up a play, but then he stopped. How do you back up a three-run homer?

Staked to a 3-0 bulge on Jeromy Burnitz’s first-inning three-run blast, Leiter (8-5) coughed up the lead like a rookie.

Leiter allowed a majestic one-out shot by Giambi and back-to-back two-out jacks by the red-hot Hideki Matsui and Jorge Posada.

On a night when the Met lineup gave a starter some run support against a quality opponent, the seasoned Leiter melted down like anonymous youngsters Jason Roach or Jeremy Griffiths.

The Mets were swept out of the Subway Series with a 5-3 loss.

Leiter, who lasted six innings and allowed seven hits, is near the top of the charts in all the wrong categories.

Leiter walked six Yankees to take over the National League lead in the category with 63.

The lefty’s five earned runs allowed vaults him to third in the NL with 60, behind former protege Glendon Rusch (73), who is now in the minor leagues, and St. Louis’ Brett Tomko (63).

Giambi’s homer followed Art Howe’s ejection for arguing a call at third base.

After Giambi shook up Leiter with the game-tying shot, the Met lefty retired Raul Mondesi on a diving catch by Roger Cedeno in right.

But Matsui, the star of Saturday’s doubleheader, tagged him with a pull job on a 2-and-1 pitch. Godzilla’s ninth of the year landed comfortably in the seats in right field, giving the Bombers a 4-3 edge.

Pitching coach Vern Ruhle came out to calm down the often excitable Leiter, but the visit didn’t help.

Next came Posada, and out went the baseball on another 2-and-1 offering.